Relaxed SIMD

2023-01-31

Co-champions: Marat Dukhan, Zhi An Ng

Today

• Relaxed SIMD based on profiles
• CG feedback on deterministic semantic for Relaxed madd/nmadd
• Phase 4 poll

Previously: Relaxed Mode

Positive reception by SIMD subgroup: relaxed-simd/105

Feedback by CG (2022-11-22):

co-design with Profiles, ensuring that Relaxed SIMD use case can be handled.

Full profile

Deterministic profile

Relaxed SIMD semantics

Web engines

Other environments: blockchain processing, live-migration cloud, embedded, etc

Profiles (profiles/pull/5)

Language subset that fits constraints of host environment

1. Syntactic subset (remove instructions)

2. Semantic subset (change instruction implementation)�

Full profile: no restrictions

Deterministic profile: all Relaxed SIMD instructions use deterministic semantics

Summary

Full profile supports relaxed semantics

Deterministic profile uses deterministic semantics (via Wasm SIMD)

• all Relaxed SIMD instructions have deterministic semantics

[...] only appears with !DET

Spec text changes

Syntax

Execution/Numerics

Relaxed madd/nmadd

2 options for deterministic semantics:

1. Fused Multiply-Add (FMA, no intermediate rounding)
2. Multiply + Add (with intermediate rounding)

Would like CG opinion on this.

Execution/Instructions

Binary format

Text format

Benchmarks: end-to-end

Benchmarks: operator-level

Phase 4 poll

Phase 4 requirements

✅ Two or more Web VMs implement the feature.

• V8 and SpiderMonkey

✅ At least one toolchain implements the feature.

• Emscripten/LLVM/Binaryen

✅ The formalization and the reference interpreter are usually updated (though these two can be done as part of step 3 at the Working Group chair's discretion).

• PRs uploaded (preview at https://www.ngzhian.com/relaxed-simd/core/)

✅ Community Group has reached consensus in support of the feature.

• Subgroup has reached consensus

Performance improvements